Substantial advances in screw design technology have been made in the past 15 years. A variety of special mixing sections have been designed, for example, to replace part of the metering zone of a molding screw, especially for processing polyolefins. Screw designs are also available that handle a narrow range of materials; for example, low compression screws for handling mainly amorphous resins and high compression screws for handling crystalline materials, such as high density polyethylene. Also, barrier screws with two channels to separate solid melt are now available on the market, of which the Willert screw disclosed in U. S. Pat. No. 4,330,214 is exemplary. Yet, with all of the advances that have been made, the general purpose screw remains the most common equipment installed on most injection molding machines. The general purpose screw has a profile general enough to process several materials well, but none optimally. In todays manufacturing environment of just-in-time production schedules and quick-mold-change requirements, a strong need exists in the molding industry for a molding screw that will handle a large variety of materials efficiently and without compromising quality.
At present, two different screw designs, namely extruder and injection screw designs are utilized in the molding industry. U. S. Pat. No. 3,689,182, which issued to Kovac on Sept. 5, 1972, is exemplary of an extruder screw design. In this patent, the molding apparatus utilizes a screw type extruder for reducing a solid particulate plastic material to a melt material by maintaining a spiral solid body of continuous width material in continuous contact with a hot barrel of the extruder. The melt material is separated from the solid body as rapidly as formed by a barrier screw and conducted through radial passageways into a low pressure, axially extending, cavity within the extruder screw. The extruder screw extrudes the melt material under low pressure, and, in essence, is analogous to a sausage grinder in which the rotating screw transports chunks of meat through a cutter and die at low pressure.
The known injector screw design differs from the known extruder screw in that the melt material is subjected to a maximum pressure as high as 20,000 psi, for example, for forcing the melt material into an injection mold. This was achieved by modifying the outer screw so that it could be reciprocally moved within the barrel to function as a hydraulic ram for injecting a predetermined volume of the melt material into a mold. In this known injection screw design, the outer screw is normally nonrotatable during the injection stroke of the screw. This is so because rotation of the screw would cause a serious problem, namely the over-heating of the melt material in the passages between the periphery of the screw and the barrel causing degradation of the melt material resulting in an inferior product. Some of the earlier screw designs during the 1960's had the ability to rotate the screw during injection, but the screw design was discontinued because of the aforementioned over-heating problem. In an attempt to minimize this serious problem, the injection screw was normally provided with a non-return valve at the metering end of the screw. The non-return valve seals passages in the end of the screw during its movement in a forward direction to prevent the melt material from being forced back into the screw, by-passing the screw thread and overheating. However, the non-return valve presented an added serious and expensive maintenance problem because the parts of the non-return valve are subjected to extensive wear due to the action of the corrosive and abrasive melt material passing therethrough. Such wear of the non-return valve parts and between the non-return valve and barrel would result in a premature wear-out of the valve parts and an increase in barrel diameter necessitating frequent and costly replacement of the non-return valve parts and barrel.
The aforementioned serious problems presented by an extruder screw modified to function as an injector screw are obviated by the improved injector screw design of this invention which reliably and efficiently processes a large variety of plastic materials in both the injector and extruder modes of operation.
Accordingly, a purpose of the present invention is to provide an improved injector screw that will process a large variety of plastic materials efficiently and without compromising quality. The injection screw can also function superbly as an extruder screw merely by retaining the outer screw in its forward position.